1. Field of the Invention
The present invention relates to a surface mount type quartz crystal oscillator with a reduced height dimension, and in particular, to a surface mount type crystal oscillator mounted in a thin electronic card or a smart card such as a SIM (Subscriber Identify Module) card, a PC (Personal Computer) card, or an IC (Integration Circuit) card.
2. Description of the Related Arts
An SIM card, used in a cellular phone, is a typical example of an electronic card composed of a small card containing a memory device and an IC chip. The SIM card stores authentication information and the like for the cellular phone and is used to store use permission information for the cellular phone and personal information such as a telephone number. In recent years, efforts have been made to incorporate a GPS (Global Positioning System) function into the SIM card. In this case, a precise frequency reference signal needs to be supplied to a GPS receiver circuit. Thus, the card needs to include a temperature compensated crystal oscillator of a surface-mount type with a height, i.e., a thickness; reduced so as to accommodate the SIM card. The SIM card is, for example, 0.76 mm in thickness. Thus, the surface-mount type crystal oscillator contained in the SIM card needs to have a thickness of 0.5 to 0.4 mm or less.
The surface-mount type crystal oscillator is composed of a quartz crystal blank and an IC chip with an oscillation circuit that uses the crystal blank; the crystal blank and the IC chip are accommodated in a surface-mount type container. The surface-mount type crystal oscillator is built in various portable devices as a reference source for frequency or time. The surface-mount type temperature compensated crystal oscillator is one kind of surface-mount type crystal oscillators which includes a temperature compensating mechanism built in the IC chip to compensate for frequency-temperature characteristics of the crystal blank.
In general, surface-mount type crystal oscillators are classified into, for example, a one-chamber type, a two-chamber type (i.e., H-shaped cross section type), and a bonding type. In a one-chamber type crystal oscillator, the crystal blank and the IC chip are hermetically encapsulated in a single recess formed in a container body. FIG. 1A is a sectional view of an example of a configuration of a conventional surface-mount type temperature compensated crystal oscillator of the one-chamber type. FIG. 1B is a plan view of an IC chip used in the crystal oscillator.
In the illustrated one-chamber type crystal oscillator, a recess is formed in one principal surface of flat, generally parallelepipedic container body 1 which is made up of laminated ceramics. A step portion is formed on an inner wall of the recess. Pair of holding terminals 10 is provided on a top surface of the step portion of the recess to hold crystal blank 3. IC chip 2 and crystal blank 3 are accommodated in the recess. In the crystal oscillator, metal ring 13 is provided on an opening end surface of container body 1, that is, a top surface which surrounds the recess. Metal cover 14 is seam-welded to metal ring 13 to close and seal crystal blank 3 and IC chip 2.
IC chip 2 has a generally rectangular planar shape, and includes at least an oscillation circuit and a temperature compensating mechanism integrated therein. The oscillation circuit uses crystal blank 3 and the temperature compensating mechanism compensates for frequency-temperature characteristics of crystal blank 3. In IC chip 2, electronic circuits are formed on one principal surface of a semiconductor substrate by a normal semiconductor device fabrication process. One of both principal surfaces of the semiconductor substrate on which the electronic circuits are formed is called a circuit forming surface. A plurality of, in this case, three IC terminals 4, used to connect IC chip 2 to an external circuit, are provided on the circuit forming surface of IC chip 2 along each long side thereof. IC terminals 4 are formed as, for example, conductive pads. IC terminals 4 include a power supply terminal (Vcc), an output terminal (OUT), a ground terminal (GND), and an AFC terminal, as well as a pair of crystal IC terminals 4x, 4y used for electric connection to crystal blank 3. Among IC terminals 4, the power supply, output, ground, and AFC terminals are arranged at positions corresponding to four corners of the circuit forming surface. Each of crystal IC terminals 4x, 4y provided at positions corresponding to a central region of a corresponding one of two long sides of the circuit forming surface.
Plurality of circuit terminals 6 are provided on an inner bottom surface of a recess of container body 1 in association with IC terminals 4. IC chip 2 is secured to the inner bottom surface of the recess by performing ultrasonic thermocompression bonding using bumps 5 as flip chip bonding, to join IC terminals 4 to circuit terminals 6.
Mounting terminals 7, used to surface-mount the crystal oscillator on a wiring board, are provided in four corners of an outer bottom surface of the container body 1. Among IC terminals 4, the power supply, output, ground, and AFC terminals are electrically connected to mounting terminals 7 via a lamination plane between ceramic layers of container body 1 by means of a conductive path (not shown) formed in container body 1. Crystal IC terminals 4x, 4y are electrically connected to pair of holding terminals 10 by the conductive path (not shown) formed on container body 1.
As shown in FIG. 2B, crystal blank 3 is, for example, a generally rectangle AT-cut quartz crystal blank including excitation electrodes 8a, 8b formed on respective principal surfaces. Extraction electrodes 9a, 9b extend from excitation electrodes 8a, 8b toward opposite sides of one end of crystal blank 3. Each of extraction electrodes 9a, 9b is folded back between the opposite principal surfaces of crystal blank 3 at the position of the end of crystal blank 3. Crystal blank 3 is held in the recess of container body 1 and electrically connected to IC chip 2 by securing extraction electrodes 9a, 9b to holding terminals 10 by conductive adhesive 11 at positions where extraction electrodes 9a, 9b are extracted. In an operation of securing crystal blank 3, conductive adhesive 11 is applied only onto holding terminals 10. Thus, no conductive adhesive is present on a top surface, in FIG. 2B, of crystal blank 3.
A pair of inspection terminals 12 are provided on an outer side surface of container body 1. Holding terminals 10 are also electrically connected to inspection terminals 12. Inspection terminals 12 are used to measure vibration characteristics of crystal blank 3 per se. The vibration characteristics include, for example, crystal impedance (CI). Inspection terminals 12 are formed on an end surface of each ceramic layer constituting container body 1. However, in container body 1 composed of a plurality of laminated ceramic layers, no inspection terminals 12 are formed on the end surfaces of the uppermost and lowermost layers in order to prevent electric short-circuiting to metal rings 13 or the wiring board. Thus, the length of inspection terminal 12 is smaller than the height of container body 1 in a height direction of the crystal oscillator.
In the above description, circuit terminals 6, mounting terminals 7, holding terminals 10 and inspection terminals 12 are each provided as an electrode layer formed on a surface of the corresponding laminated ceramic layer.
In the two-chamber type crystal oscillator, the crystal blank is hermetically encapsulated in a recess formed in one of the principal surfaces of the container body. The IC chip is accommodated in a recess formed in the other principal surface. In this case, the container body has an H-shaped cross section. The mounting terminals are provided in the four corners of a surface of the container body which surrounds the recess with the IC chip accommodated therein. The inspection terminals are provided on the outer side surface of the container body, and alternatively the bottom surface of the recess with the IC chip accommodated therein.
The bonding type crystal oscillator is constructed by joining a mounting substrate in which the IC chip is accommodated and which includes the mounting terminals, to a crystal unit composed of the crystal blank hermetically encapsulated in a container. IC chip 2 is connected to the crystal blank and the mounting terminals as described above. In this case, terminals used to join the crystal unit to the mounting substrate may also be used as inspection terminals. Japanese Patent Laid-Open Application No. 2002-330027 (JP-2002-330027A) discloses an example of the bonding type crystal oscillator in which an assembly constructed by connecting the IC chip to a lead frame is joined to the crystal unit.
For any of the one-chamber, two-chamber, and bonding type crystal oscillators of surface-mount type, when the crystal oscillator is configured into a temperature compensated crystal oscillator, mounting terminals 7 such as the power supply terminal and the AFC terminal are used as write terminals to write temperature compensation data to the temperature compensating mechanism in the IC chip. By writing the temperature compensation data corresponding to the frequency-temperature characteristics of the crystal blank to the temperature compensating mechanism, a variation in frequency caused by the crystal blank in association with a variation in temperature can be compensated for. The write terminals may be provided on the outer surface of the container body separately from the mounting terminals.
However, in any type of surface-mount type crystal oscillator configured as described above, the IC chip and crystal blank 3 are arranged along the height direction of the crystal oscillator. Thus, a lower limit on the height dimension of the crystal oscillator is about 0.8 mm. The above-described surface-mount type crystal oscillators are unsuitable for the SIM card, which needs to be about 0.5 mm or less in height.
Japanese Patent Laid-Open Application No. 9-83248 (JP-9-083248A) discloses that the height dimension of the crystal oscillator can be reduced by arranging IC chip 2 and crystal blank 3 on the bottom surface of the recess of the container body and in juxtaposition in a horizontal direction. FIGS. 2A and 2B are a sectional view and a plan view of the crystal oscillator in which IC chip 2 and crystal blank 3 are arranged, in a horizontal direction, on the inner bottom surface of the recess formed in container body 1. No step portion is formed on an inner side surface of the recess of container body 1. A pair of holding terminals 10 are provided directly on the inner bottom surface of the recess.
The IC chip and crystal blank 3 are thus arranged on the inner bottom surface of the recess of the container body and in juxtaposition in the horizontal direction. Then, the height dimension of the crystal oscillator can be reduced down to about 0.5 mm in view of the thickness of the IC chip including the bumps, the distance between the IC chip and the metal cover, and the thickness of the metal cover per se. Instead of the flip chip bonding technique using the bumps, wire bonding may be used to electrically connect the IC chip to the container body. Also in this case, the height dimension of the crystal oscillator can be reduced to about 0.5 mm. However, either with the wire bonding or using the bumps, the conventional technique has difficulty in reducing the height of the crystal oscillator to less than 0.5 mm.
Furthermore, the above-described reduced height dimension of the crystal oscillator reduces the height of the container body per se and the size of inspection terminals 12 formed on the outer side surface of the container body, particularly the length of inspection terminals 12 along the height direction of the container body. The reduced size of inspection terminals 12 hinders a probe from a measuring instrument from abutting against inspection terminals 12.
Japanese Patent Laid-Open Application Nos. 2003-32042 and 2003-51719 (JP-2003-032042A and JP-2003-051719A) disclose real-time clock modules including a U-shaped crystal blank and an IC chip driving the crystal blank, the clock module using a container made up of laminated ceramics and including two recesses formed in one principal surface. The crystal blank is accommodated in one of the recesses, the IC chip is accommodated in the other recess, and the container and the IC chip are electrically connected together by wire bonding. According to JP-2003-32042A, both recesses are covered. According to JP-2003-51719A, the recess with the crystal blank accommodated therein is covered, whereas the recess with the IC chip accommodated therein is sealed with mold resin. Thus, neither of the real-time dock modules includes a pair of inspection terminals used to measure the vibration characteristics of the crystal blank per se.